1. Field of the Invention
The present invention relates to a balanced-unbalanced converting (balun) circuit and a laminated balanced-unbalanced converter.
2. Description of the Related Art
Generally, merchant or marchand baluns and transformer baluns having a filter for use at high frequencies, such as microwave frequencies, are known. A merchant balun is described in, for example, Japanese Unexamined Patent Application Publication No. 9-260145.
FIG. 8 shows a balun circuit 1 of this type having strip lines 2 to 5. The strip line 2 is electrically connected between a balanced terminal 6 and a ground terminal G, and the strip line 3 is electrically connected between a balanced terminal 7 and the ground terminal G. The strip lines 4 and 5 are electrically connected in series between an unbalanced terminal 8 and an open terminal 9. The strip lines 2 and 4 are opposed to each other in such a manner that a first end 2a of the strip line 2 connected to the balanced terminal 6 is diagonally opposite to a first end 4a of the strip line 4 connected to the unbalanced terminal 8, and the opposing strip lines 2 and 4 are electromagnetically coupled. The strip lines 3 and 5 are also opposed to each other in such a manner that a first end 3a of the strip line 3 connected to the balanced terminal 7 is diagonally opposite to a first end 5a of the strip line 5 connected to the open terminal 9, and the opposing strip lines 3 and 5 are electromagnetically coupled.
In a narrow-band balun circuit, capacitors C1 to C4 indicated by imaginary lines in FIG. 8 are electrically connected between the ground terminal G and the balanced terminals 6 and 7, the unbalanced terminal 8, and the open terminal 9, respectively.
FIG. 9 shows another known balun circuit 11 having four LC parallel resonant circuits 12 to 15. In FIG. 9, the balun circuit 11 includes resonant capacitors C1 to C4, resonant coils L1 to L4, coupling capacitors Cs1 to Cs6, balanced terminals 16 and 17, an unbalanced terminal 18, an open terminal 19, and a ground terminal G.
However, the balun circuits 1 and 11 shown in FIGS. 8 and 9 have problems.
The balun circuit 1 shown in FIG. 8 has an attenuation characteristic in the vicinity of the transmission band and an attenuation characteristic for suppression of spurious signals, such as second and third harmonic suppression, but the amount of attenuation is limited. Thus, an external filter, such as a low-pass filter or a band-pass filter, is additionally required for an attenuation characteristic in the vicinity of the transmission band and a spurious-signal suppression function. This increases the circuit size, the number of circuit elements, and the insertion loss.
The balun circuit 11 shown in FIG. 9 has the LC parallel resonant circuits 12 to 15, and therefore requires a large number of elements, resulting in increased insertion loss.